Single-shot diffusion-weighted echo planar imaging (EPI) is commonly used because of its robustness to motion and high signal to noise ratio (SNR) efficiency. However, the long echo-train readout and narrow effective bandwidth in the phase encode direction make this technique extremely sensitive to T2*-induced decay (blurring) and off resonance, with field inhomogeneities, eddy currents and chemical shift often resulting in severe geometrical distortion. The need for extensive anatomical coverage and high resolution makes EPI-based acquisitions particularly challenging, especially at 3 T, where the increased sensitivity to off-resonance and B1 inhomogeneity cause increased anatomical distortion and shading.
Several high-resolution DWI methods based on echo-planar trajectories have been developed that maximize the velocity of k-space traversal in the phase encode direction to limit distortion and blurring while preserving resolution. Parallel imaging is used extensively with DWI to reduce the effective encoded field of view (FOV), however, coil geometry, noise amplification and residual aliasing limit practical acceleration factors when the FOV is on the order of the phased-array element size or smaller. In multi-shot methods, only a sub-set of k-space lines are acquired following each diffusion-sensitizing period, so that several acquisitions are necessary to fully encode k-space. Additional navigator data or carefully designed, self-navigated trajectories are therefore necessary to correct for shot-to-shot phase inconsistencies caused by physiological and bulk motion. Despite their complexity, the small FOV effectively encoded and short echo spacing achievable with these techniques has been shown to provide excellent resolution and anatomical fidelity, especially in the brain.
Single-shot EPI has been used in conjunction with outer volume suppression pulses, inner volume excitation and 2D RF pulses for high resolution imaging of targeted regions. Some of these methods have been successfully used for imaging the spine, prostate, pancreas, kidneys and thyroids as well as for treatment monitoring in the breast. Due to their limited coverage these techniques are unsuitable for screening purposes. Several groups have recently explored strategies to perform image-space combination of a series of reduced FOV images consecutively acquired to cover the desired FOV. Estimates of the excitation profiles and generalized parallel imaging reconstruction techniques have been shown to allow smooth combination of contiguous volumes with minimal overlap. The main limitation of these methods is that they require a large number of acquisitions to cover the prescribed FOV, which severely limits their applicability outside the brain, where much larger FOVs are often used.
A method that retains the high resolution and anatomical fidelity offered by reduced FOV techniques while extending spatial coverage with the help of generalized parallel imaging concepts could be very beneficial to EPI-based MR applications such as single shot diffusion-weighted EPI.